Three closely related, midwinter to spring-flowering, mesic species of Zephyranthes (Amaryllidaceae) are endemic to the southeastern United States: Z. atamasca, Z. treatiae, and Z. simpsonii. The taxonomic level of the distinctiveness of Z. treatiae from Z. atamasca is discussed. Based on herbarium, cytological, and field studies, and on Principal Component Analysis and pictorialized scatter diagram analysis, we conclude that all three taxa are distinct at the species level. They are distinguished by differences in relative lengths of floral elements. Illustrations, a distribution map, and a key are provided.